7. COSMOLOGICAL IMPLICATIONS

A key question that remains is the position of compact groups in the
clustering hierarchy. Are compact groups distinct entities
(Sulentic 1987) or
an intermediate stage between loose groups and triplets, pairs and individual
galaxies (Barnes
1989,
White 1990,
Cavaliere et
al. 1991,
Rampazzo & Sulentic
1992,
Diaferio et
al. 1994). Some compact groups are purely
projection effects, others may be small clusters (Ebeling et al.
1995), but most appear to be real.
It seems that they can arise naturally from subcondensations in looser groups,
but further studies are needed to better determine both the observed space
density of groups as a function of population and the timescales involved in
the evolutionary process.

This question is related to that of the formation mechanism of compact groups.
Two mechanisms have been discussed in the literature. Diaferio et al.
(1994)
conclude that compact groups form continually from bound subsystems within
loose groups. This gains some support from the observation (see
Section 3)
that most HCGs are embedded in loose groups, although it is not obvious that
these loose groups are sufficiently rich (Sulentic 1997).
Governato et
al. (1996), proposed a model in which merging activity in
compact groups is accompanied by infall of galaxies from the
environment. This naturally explains
the observed mix of morphological types, and it allows compact groups to
persist for longer times.

Where do the Shakhbazian groups fit in this picture? Recent studies
(Tikhonov 1986,
Amirkhanian &
Egikian 1987,
Amirkhanian et
al. 1988,
1991,
Amirkhanian 1989,
Kodaira et
al. 1988,
1990,
1991,
Stoll et al.
1993a,
1993b,
1994a,
1994b,
1996a),
1996b
show that these objects
are typically compact clusters or groups of early-type galaxies. Although the
systems were selected on the basis of red colors and compact appearance of
their galaxies, both of these factors result from their large distances
because K-corrections and contrast effects become significant. The galaxies
are in fact relatively normal, although luminous (Del Olmo et al.
1995).
However, the number of blue (gas rich) galaxies in these systems does seem to
be very small. Thus it appears the Shakhbazian groups are mostly small
clusters, possibly intermediate in physical properties between classical
compact groups and clusters.

If the groups are dynamically bound, galaxy mergers should commence within a
few dynamical times (Carnevali et al. 1981,
Ishizawa et
al. 1983,
Barnes 1985,
Ishizawa 1986,
Mamon 1987,
1990,
Zheng et al.
1993). Both N-body and
hydrodynamic simulations indicate that the dark matter halos of individual
galaxies merge first, creating a massive envelope within which the visible
galaxies move (Barnes 1984,
Bode et al.
1993). Kinematic studies of loose groups
(eg. Puche &
Carignan 1991) indicate that the dark matter is concentrated
around the individual optical galaxies. In contrast, the X-ray observations
indicate that in most compact groups, the gas and dark matter is more extended
and is decoupled from the galaxies. This may explain the observation that
galaxies in compact groups typically have mass-to-light ratios 30% to 50%
lower than more isolated galaxies (Rubin et al. 1991).

Is there any observational evidence that galaxies in compact groups are
merging? By 1982 it was evident that first-ranked galaxies in compact
groups did not appear to be merger products, because the fraction of
first-ranked galaxies that are type E or S0 is the same as for the general
population of HCG galaxies (Hickson 1982). If mergers were a dominant effect,
the first-ranked galaxies would be expected to be more often elliptical.
The same conclusion was reached by Geller & Postman (1983) who found that the
luminosities of first-ranked galaxies were consistent with a single luminosity
distribution for all group galaxies. Of course this may just mean that
in small
groups the first-ranked galaxy is not necessarily the most
evolved. Rather, one should ask if any galaxies in compact groups
show indications of merging.
The relative paucity of merging galaxies in compact groups was first noted by
Tikhonov (1987),
from a visual inspection of optical images. Zepf & Whitmore
(1991) realized that elliptical galaxies formed by recent mergers of
gas-rich
systems should have bluer colors than normal. Examining the HCGs, they found
only a small enhancement in the fraction of early-type galaxies having blue
colors, a conclusion reinforced by an independent study by
Moles et al.
(1994).
On the other hand, Caon et al. (1994) argued that the large
effective radii of compact group elliptical galaxies is indicative of an
origin by merging or accretion of companions.

Zepf (1993)
estimated that roughly 7% of the galaxies in compact groups are
in the process of merging. This conclusion was based on roughly consistent
frequencies of (a) optical signatures of merging, (b) warm
far-infrared colors, and (c) sinusoidal rotation curves. However, few
galaxies show all of these effects simultaneously. The merging fraction may
thus be as high as 25% if one allows that any one of these criteria would be
considered to be sufficient to indicate a merger (Hickson 1997). Given the
small numbers of objects in these studies, it is fair to say that the fraction
of merging galaxies is highly uncertain at present. It seems safe to conclude
that current observations do not rule out a significant amount of merging in
compact groups.

Detailed studies of individual compact groups can be quite revealing. Many
galaxies that at first appear normal are revealed to have peculiar morphology
or spectra when examined more closely. Many, perhaps most, compact groups
clearly contain galaxies that are dynamically interacting. However, the groups
likely span a range of evolutionary states. At the extreme end are
high-density
groups like Seyfert's Sextet, HCG 31, HCG 62, HCG 94 (Pildis 1995) and HCG 95
(Rodrigue et
al. 1995) in which we find strong gravitational
interactions. At the other end are lower density compact groups, such as
HCG 44, which most
likely are in a less-advanced stage of evolution. This picture is supported by
radio observations: Seyfert's Sextet and HCG 31 are both embedded in
extended
HI clouds whereas in HCG 44 the HI is associated with individual galaxies
(Williams et
al. 1991).

It seems clear that the groups as we now see them can persist for only a
fraction of a Hubble time. Simulations indicate that merging should
destroy the
group on a time scale tm that is typically an order of
magnitude larger
than td, (Cavaliere et al. 1983,
Barnes 1984,
Navarro et
al. 1987,
Kodaira et
al. 1990), although longer lifetimes are possible depending on the
distribution of dark matter (Athannasoula et al. 1997) and
initial conditions
(Governato et
al. 1991). Assuming that the groups are in fact bound dynamical
systems, we can draw two conclusions: (a) There must be an ongoing
mechanism for forming or replacing compact groups, and (b) there
must be a significant population of relics of merged groups.

What are the end-products of compact groups? It is tempting to identify
them with field elliptical galaxies, following a suggestion first made
by Toomre
(1977). Simulations (Weil & Hernquist 1994) indicate that multiple
mergers in small groups of galaxies best reproduce the observed kinematical
properties of elliptical galaxies. The resulting galaxies are predicted to
possess small kinematic misalignments, which can be detected by detailed
spectroscopic and photometric studies. Neverthess, it remains to be
demonstrated that these merger remnants can reproduce the tight correlation
between size, luminosity and velocity dispersion found in present-day
elliptical galaxies.

If compact groups have lifetimes on the order of tm,
and form continuously,
then the number of relics, per observed group, is expected to be on the order
of (H0tm)-1. Thus, the
number of relics could exceed that of present day
groups by as much as an order of magnitude. Mamon (1986)
estimated that, if all
HCGs are real, then the relics would account for about 25% of luminous field
elliptical galaxies. As we have seen, the true space density of compact groups
is uncertain by at least a factor of two, and may be underestimated because of
selection biases. There is then the potential problem of producing too many
relics.

A second problem is the fact that the integrated luminosities of compact
groups
are typically a factor of three to four times greater than luminosities of
isolated elliptical galaxies (Sulentic and Rabaça 1994). It is possible
that interaction-induced star formation has boosted the luminosities of some
compact-group galaxies, and that some degree of fading of the merger product
is expected. However, at this point it is not clear whether or not the relics
can be identified with isolated elliptical galaxies.

Despite these problems, a fossil compact group may have actually been found.
Ponman et al.
(1994) have detected a luminous isolated elliptical galaxy
surrounded by diffuse X-ray emission which is consistent with the expected
end-product of a compact group. If more objects like this are found, it may be
possible to compare their space density with that expected for compact group
relics.

Interactions are often implicated in the development of active nuclei in
galaxies (eg. Freudling & Almudena Prieto 1996). The HCG catalog
includes
several examples of compact groups containing both starburst galaxies and AGN.
Several recent examples of associations between starburst galaxies or AGN and
what appear to be compact groups have been reported: Del Olmo & Moles
(1991)
have found a broad-line AGN in Shakhbazian 278; Zou et al.
(1995) find that
the luminous infrared source IRAS 23532 coincides with a compact group that
includes a Seyfert 1 as well as a starburst galaxy. If this association
extends to QSOs, one
would expect to find numerous compact groups at redshifts z ~ 2,
where the comoving number density of QSOs peaks (eg. Hartwick & Schade
1984). The tendency for QSOs to have close companions has been known
for some time (eg. Stockton 1982,
Bahcall et
al. 1997). Recently, several examples of
compact groups associated with luminous infrared galaxies, AGN and QSOs at
z 2 have been
found using HST (Pascarelle et al. 1996, Francis et al.
1996,
Matthews et
al. 1994,
Tsuboi & Nakai 1994,
Hutchings 1995,
Hutchings et
al. 1995).

These observations provide support to the idea that tidally-triggered star
formation is a predominant factor in the galaxy formation process
(Lacey & Silk 1991,
Lacey et al.
1993). In this model disk star formation
occurs relatively late, after the compact group has formed and tidal
interactions are strong. This seems at least qualitatively consistent with the
fragmentary nature of high-redshift galaxies observed with the Hubble Space
Telescope (Schade
et al. 1995), although these fragments appear to be
much less luminous and more irregular than most present-day compact group
galaxies. The model also offers a possible explanation for the excess numbers
of faint blue galaxies found in field galaxy count as dwarf galaxies
undergoing star formation at a redshift of z 1.

Compact groups may possibly play a role in the formation of other systems. We
have seen that giant galaxies may be formed as the end product of
compact-group
evolution. At the other end of the scale, dwarf galaxies have physical
properties distinct from normal galaxies, which suggests a unique formation
mechanism. One possibility is that they form during gravitational interactions
from tidal debris (Duc & Mirabel 1994). If this is the case, one would expect
to find evidence for this in compact groups of galaxies. From an
examination of condensations in tidal tails, Hunsberger et
al. (1996) concluded that the
fraction of dwarf galaxies produced within tidal debris in compact groups is
not negligible. There is also evidence that star clusters form from tidal
debris. Longo et
al. (1995) have found an excess population of unresolved blue
objects around HCG 90 which appear to be recently formed star
clusters. These
may be similar to the population of new star clusters recently reported in the
merger remnant NGC 7252 (Whitmore et al. 1993).

Because compact groups have a high galaxy surface density, they may form
effective gravitational lenses. Gravitational amplification of background
field galaxies was proposed by Hammer and Nottale (1986) as a possible
explanation for the presence of the high-redshift discordant member of this
group. Mendes de
Oliveira and Giraud (1994) and Montoya et al. (1996) find that
most HCGs are too nearby to produce strong lensing effects. However, because
the critical mass density required for strong lensing depends reciprocally on
distance, analogous systems 5-10 times more distant should produce a
non-negligible fraction of giant arcs.

Studies of small groups may provide clues to the overall structure of the
universe. The baryon fractions found in clusters of galaxies appear to be
inconsistent with a density parameter = 1, unless the dark matter is
more prevalent outside clusters (White 1992, Babul & Katz 1993). Compact
groups provide a means to study dark matter in such regions. The baryon
fractions found for compact groups do appear to be lower than those for
clusters. David
et al. (1995) argued that the gas is the most extended component;
galaxies being the most compact and the dark matter being intermediate. They
concluded that the baryon fraction approaches 30% on large enough scales,
which is comparable to the values found for clusters. Given the constraints of
standard Big-Bang nucleosynthesis this would imply that the density parameter
is at most 0.2. On the
other hand, the infall picture of compact-group
evolution (Governato
et al. 1996) requires a high-density ~ 1
universe. In a low-density universe the infall rate is insufficient. As there
is at present no other clear mechanism for avoiding the overproduction of
relics by merging compact groups, this may be a strong argument for a
high-density universe.

During the last two decades we have seen a resurgence of interest in compact
groups. While initially little more than a curiosity, these systems are now
viewed as potentially important sites of dynamical evolution, shaping the
structure of many galaxies. It now seems clear that while many compact groups
are contaminated by projections, a large fraction of at least the
high-surface-brightness HCGs are physically dense. They form by gravitational
relaxation processes within looser associations of galaxies. The densest are
generally in an advanced stage of evolution characterized by strong
interactions, starburst and AGN activity, stripping of stellar and dark matter
halos, and merging. They contain large amounts of dark matter and primordial
X-ray-emitting gas trapped within the gravitational potential well.

Despite this progress, many questions remain unanswered. What are the end
products of compact group evolution, and do they have properties consistent
with any know population of objects? What is the space density of such relics?
Where do compact groups fit in the overall clustering hierarchy? What is
their role in the evolution of galaxies both past and present? Given the
current interest and research activity in this area, it is likely that
many of these questions may soon be addressed.
ACKNOWLEDGEMENTS

It is a pleasure to thank the Observatories of Brera and Capodimonte for
hospitality during the initial work on this review. I have benefitted from
discussions with many individuals, but I would like to acknowledge
particularly
the contributions of A Iovino, E Kindl, G Longo, G Mamon, C Mendes de Oliveira,
TK Menon, G Palumbo, H Rood, and J Sulentic. I thank G Mamon, A Sandage
and J Sulentic for providing helpful comments on an earlier version of the
manuscript. Financial support was provided by the Natural Sciences and
Engineering Research Council of Canada and NATO.